Power transformers need to withstand the dynamic effects of short circuit without damage. However during a short circuit, the electromagnetic forces on the winding package of a power transformer are very large. Therefore, the windings are clamped to resist the electromagnetic forces during a short circuit. Due to the material ageing and shrinking, repeated thermal cycling, short circuits and so on, the clamping forces decrease during the lifetime of the transformer. A decreased clamping force will consequently reduce the short circuit withstand capability of a power transformer. Therefore it is important to assess how rigidly the winding packages are clamped to assure the short circuit withstanding capability of a transformer.
To determine the clamping forces of a winding package of a power transformer, conventionally the transformer interior is inspected, for example by measuring the torque of the bolts of the press plate, which however requires that the transformer be put out of service for a considerable time while the transformer has to be drained from oil and often the active part has to be lifted out of the tank. Additionally, there is always a risk of contamination whenever a transformer tank is opened.
It is known that the strongly non-linear strain-stress characteristic of the pressboard used in the transformer insulation gives rise to a mechanical resonant frequency that is dependent on the clamping forces on the windings. If the clamping force is reduced the mechanical resonant frequency is shifted to lower frequencies. Thus by determining the mechanical resonant frequency the clamping forces can be determined.
A patent SU1390643 has disclosed a method for inspecting the quality of impregnating windings of electrical devices, such as motors, transformer coils, chokes, etc. The inspecting is based on the physical phenomenon of vibration generated by passing a pulse current through the winding, thereon the quality of the winding is evaluated from vibration signal.
A numerical method for determining the mechanical resonant frequency of windings can be found in a paper, entitled “Dynamic response of power transformers under axial short circuit forces, Part 2—Winding and clamps as a combined system”, IEEE Transactions on Power Apparatus and Systems, Volume PAS-92, No 5, September-October 1973, pp. 1567-1576. This method presents a numerical solution, or a mathematical model to calculate the mechanical resonant frequency of the windings, the calculated resonant frequencies then being compared with tested ones. The paper has acknowledged that the non-linear strain-stress of pressboard used in the transformer insulation gives rise to the mechanical resonant frequency of winding that depends on clamping forces. However, the method is based on a mathematical model to calculate the mechanical resonant frequency of windings rather than a measurement directly on site. This means that a verification of the model has to be carried out. However, this is not practical for a transformer on site since it is based on coils in a laboratory as the used technique.
Although it is suggested that the clamping force can be determined by analysis of transformer vibrations, and the mechanical resonant frequency of the winding is known to be dependent on clamping forces on the windings of a transformer, no useful and practical method to determine the mechanical resonant frequency of a winding package of a transformer exists today.
It is also of general interest to know how much a transformer or a reactor sounds. One way to find it out is to determine the resonant frequencies of the transformer or the reactor. There is a need to determine the resonant frequencies in a practical way, for example on site and without complicated operations.